Hello,
I just wanted to ask if anyone would happen to know some easy and fast to use editor for creating and test user interfaces.
It would need to have the ability to implement pop-up-menues and display tooltips.
Thanks in advance.

Really depends on which language you're writing your application in.
Chances are though that you're writing a language that the Qt Framework supports, which has one of the best UI editors/designers there is. It's pretty complex, but it can make damn good looking UIs.